As a result of the elimination, mainly for ecological reasons, of lead alkyls in gasoline, the isomerization of normal paraffins having 4 to 6 carbon atoms is at present of considerable importance in the petroleum industry.
The isomerization of n-butane makes it possible to produce isobutane for the aliphatic alkylation of olefins and the synthesis of MTBE (methyl tert. butyl ether), respectively making it possible to produce an alkylate having a high octane number which can be incorporated into the gasoline pool and supply MTBE used as an octane additive.
The isomerization of normal C.sub.5 -C.sub.6 paraffins makes it possible to transform these low octane number paraffins into high octane number isoparaffins.
Three different types of catalysts are conventionally used for carrying out this isomerization reaction:
Friedel-Crafts catalysts, such as aluminium chloride, which are used at low temperatures (approximately 20.degree. to 130.degree. C.);
catalysts based on metals of group VIII on halogenated alumina, which are used at medium temperatures (approximately 150.degree. C.);
zeolitic catalysts incorporating a metal of group VIII deposited on a zeolite and which are used at high temperatures (250.degree. C. and higher);
these catalysts leading to reduced octane number improvements, but have the advantage of being easier to obtain and are more resistant to poisons, but as a result of their low acidity, they cannot be used for n-butane isomerization.
Numerous patents deal with monometallic catalysts based on platinum deposited on a halogenated alumina, said catalysts being used in the isomerization of normal paraffins, in which the molar hydrogen/hydrocarbon (H.sub.2 /HC) ratios are relatively high, in order to prevent deactivation by coking linked with secondary reactions or the temporary presence of sulphur, cf. e.g. U.S. Pat. No. 2,906,798, in which H.sub.2 /HC is higher than 0.17, U.S. Pat. No. 2,993,398 and U.S. Pat. No. 3,791,960, in which H.sub.2 /HC is respectively between 0.2 and 10 and between 0.1 and 15, as well as U.S. Pat. No. 4,113,789 and U.S. Pat. No. 4,149,993, in which H.sub.2 /HC is approximately 0.2.
More recently, U.S. Pat. No. 4,804,803 claims the use of a monometallic catalyst having a greater resistance to deactivation, more particularly in the presence of sulphur, in a process for the isomerization of normal paraffins and without hydrogen recycling, using H.sub.2 /HC molar ratios in the effluent of below 0.05. These low ratios minimize the costs of the process, because they require no hydrogen separation apparatus or compressor for the recycling thereof.